Pack cementation is a well known chemical vapor diffusion technique for applying diffusion coatings to metal surfaces. This process involves placing a pack mixture into close contact with the surface being coated and subsequently heating the entire assembly to an elevated temperature for a specified period of time. During heating the coating material diffuses from the pack onto the surface of the metal by a combination of chemical reactions and gas phase mass transport. A common pack mixture used to create a chromium coating contains chromium, an inert filler such as alumina, and a halide activator. Davis in U.S. Pat. No. 4,904,501 teaches that ammonium chloride, sodium chloride and ammonium bromide can be used as activators. Clark et al. in U.S. Pat. No. 3,779,729 disclose a diffusion coating for jet engine components applied using a pack mix. Depending upon the desired coating the pack may contain aluminum, chromium, silicon or combinations of these elements. The reference further teaches that a trace amount of activator ranging from 0.1 to 3 percent by weight be used. According to the patent this activator is generally a halogen or halogen precursor compound. Fluorine, chlorine, bromine and iodine per se and in salt form, particularly alkali and alkaline earth metal and ammonium salt forms are said to be examples of acceptable activators. However this patent does not teach or suggest that any one activator would perform differently from another and does not even identify the halogen activator used in the examples. The most common practice followed by Davis, Clark et al. and others is to use a single activator.
Some dual chloride and fluoride activator systems have also been proposed to enhance the ability to co-deposit both chromium and silicon. Rapp et al. in U.S. Pat. No. 5,492,727 disclose that in dual activator Cr-Si cementation packs containing a chloride and a fluoride dual activator the chlorine primarily increases the vapor pressure of chromium gaseous species, and the fluorine primarily increases the vapor pressure of silicon gaseous species. Therefore, by adjusting the ratio between chloride and fluoride in a dual activator approach, one can achieve different proportions of chromium-silicon in the coating. Yet, the data they present does not show how this might be accomplished. Rapp et al. use only one dual activator: 90MgCl.sub.2 -10 NaF. Their data shows that the amount of chromium-silicon in the coating varies among different substrates when that one dual activator is used. Rapp et al. also teach that a desired Cr-Si diffusion coating will contain 25-30 wt. % Cr and 3-4 wt. % Si. They say that to achieve that desired result requires an exact control of the fluxes of Cr and Si from the pack to the steel during the coating process. Yet, they do not teach how to perform that control. All of the pack mixes in their example contain the same ratio and amount of these elements, namely 20% Cr and 2% Si. Finally, Rapp et al. teach that a two step heating process, first at 925.degree. C. and then at 1150.degree. C. should be used. However, it is preferable to have a single heating step.
Two significant users of chromium-silicon coated metal products are the power generation industry and the petrochemical processing industry. Those users often demand chromium-silicon coated parts having a coating thickness of at least 10 mils and at least 30% chromium in the coating. As can be seen from tests reported herein, the process described in the Rapp '727 patent will not produce a coating that meets this specification.
Rapp and Harper in U.S. Pat. No. 5,364,659 disclose a pack cementation process for co-depositing chromium and silicon on an iron base workpiece using a chromium-silicon masteralloy and a dual halide activator of sodium fluoride and sodium chloride. In this process the workpiece is heated to between 800.degree. C. and 1200.degree. C. (1472.degree. F., to 2192.degree. F.) for a sufficient time to deposit a coating of chromium and silicon on the workpiece. The examples discussed in the patent were heated for 16 or 20 hours. This method is not commercially acceptable for several reasons. Use of a chromium-silicon masteralloy as a chromium and silicon source is more expensive than using pure chromium/silicon or ferrochromium/ferrosilicon. Second, our test results reported herein show that a coating thickness of 10 mils cannot be achieved and the silicon content of the resultant coating is very low. Consequently, there is a need for an effective chromium-silicon diffusion coating process which operates at a single process temperature and creates a coating of at least 10 mils thickness and containing at least 30% chromium.